A membrane in equine anatomy is a thin, pliable sheet of tissue that lines, covers, encloses, or separates structures in the body. The horse has several membranes of clinical importance, each defined by its location and function.
The synovial membrane (synovium) lines the interior of every joint capsule, tendon sheath, and bursa. It secretes synovial fluid: the viscous lubricant that reduces friction between articular surfaces and nourishes cartilage, which has no direct blood supply. Inflammation of the synovial membrane is synovitis, one of the earliest signs of joint disease; the membrane swells, overproduces fluid, and the resulting effusion is what a veterinarian palpates as joint distension. In the horse”s distal limb, where tendon sheaths are close to the surface, synovial effusion is often visible as soft, fluctuant swelling around the fetlock or pastern.
The peritoneum is the serous membrane that lines the abdominal cavity and covers most of the viscera. It is the surface across which colic pain signals travel; portions of the large colon and cecum are suspended by peritoneal folds (mesentery and mesocolon) that constrain how far displaced bowel can migrate. Peritonitis (infection or inflammation of this membrane) is a life-threatening complication of rupture or surgical contamination.
The chorioallantois (commonly called the “placenta” or “afterbirth”) is the fetal membrane that interfaces with the uterine lining during gestation. Normally expelled within three hours of foaling, a retained fetal membrane is an obstetric emergency: bacterial colonization begins within hours and can progress to laminitis and septicemia.
The pleural membrane lines each half of the thoracic cavity and the lung surfaces. Fluid accumulation between the two pleural layers (pleural effusion) dampens breath sounds and compromises lung expansion, seen in pleuropneumonia.
In clinical shorthand, the membranes almost always refers to the fetal membranes; a veterinarian asking whether the membranes have passed is inquiring specifically about the chorioallantois.